The present invention relates generally to oscillator circuits and in particular to a local oscillator architecture that reduces a transmitter pulling effect in a radio-frequency transmitter.
Radio-frequency (RF) transmitters implemented on silicon chips are commonly used in wireless communication systems such as local area networks, cellular phones, and so on. An example of a conventional RF transmitter 100 is shown in FIG. 1. Data received by transmitter 100 are processed in a signal processing system 104, which may include digital-to-analog conversion, filtering, and other processing, resulting in a data signal D. Data signal D is then used by a mixer 106 to modulate a carrier signal having a frequency f0. The carrier signal is generated by a local oscillator 108, which generally includes a voltage controlled oscillator (VCO) 110 operating at the carrier frequency f0. Ideally, local oscillator 108 provides a pure tone at frequency f0 so that all modulations in the output of mixer 106 are due to data signal D. The modulated signal D′ is amplified by a power amplifier 112, which typically consumes about 1 watt, and transmitted via antenna 114.
In silicon-chip implementations, transmitter 100 is subject to a pulling effect. The large output current from amplifier 112 can leak through the substrate or through various circuit pathways of the chip. As indicated by the dotted line in FIG. 1, this leakage current can affect the operation of VCO 110, causing the frequency spectrum of VCO 110 to be significantly wider than the ideal pure tone at f0. This pulling effect may result in undesirable frequency components in the transmitted signal, increasing the occurrence of receiver errors. The effect can be reduced to some extent by placing a buffer between local oscillator 108 and mixer 106, but it cannot be eliminated.
A more effective technique is to design the VCO to operate at a different frequency from the carrier frequency f0 and then use a combination of frequency shifter and mixer circuits to shift the frequency back to the carrier frequency f0. This technique, however, results in generation of unwanted sidebands and spurs at the mixer output, which if not addressed would result in undesirable frequency components in the transmitted signal. Bandpass filter circuits may be used to reduce the unwanted sidebands and spurs. However, filter circuits consume silicon area, increasing chip size. Moreover, for high-frequency applications, real bandpass filters are difficult to implement in silicon, due to limitations in the quality of passive components of the filter.
Therefore, an improved local oscillator architecture that reduces the pulling effect and minimizes unwanted sidebands and spurs, would be desirable.